USGS explains science behind tsunamis

March 11, 2011 6:40:00 PM PST

MENLO PARK, Calif. --

Tsunamis can have a devastating impact on low-lying coastal areas, including Northern California, thousands of miles from where the waves from the powerful 8.9 earthquake off the coast of Japan originated. But distance from the quake is a critical factor, as is the topography of the ocean floor.

The raw unstoppable power of the ocean was obvious as it let loose its fury on Japan's coast line Thursday. As the tsunami roared ashore, it took anything and everything in its path.

United States Geological Survey tsunami specialist Eric Geist has been monitoring the swell as it moved across the globe.

"Once they get going from a very large event such as a magnitude 9 or so earthquake, they are very long-lasting. They can travel all the way across the Pacific," said Geist. "The only thing that is going to stop them are land masses."

A tsunami is a series of waves created by an undersea earthquake. It happens when a quake thrusts the ocean floor upwards. That forces the water above it to rise quickly. The momentum of that upward thrust then races outward until it runs out of energy, or hits land.

As a tsunami wave travels into shallower water near the coast, it slows and grows in height. The first part of a tsunami to reach land is a trough of water called a drawback. The water near the shore moves away from the coast dramatically. However, that water doesn't stay away long, returning rapidly and creating potentially devastating surges.

Tsunamis can travel thousands of miles, taking hours to settle.

"Once they hit the coast, all kinds of other waves are generated making wave after wave after wave that will be recorded on tide gauges for many, many hours," said Geist.

An animation from the National Oceanic and Atmospheric Administration shows the impact and magnitude of the tsunami that followed the Honshu quake off Japan's coast.

"Some of the shorelines are very flat. Once the tsunamis hit those flat regions they can go very far inland," said Geist. "Some other places, like our California coast, has these substantial cliffs. If a tsunami of two meters hits those cliffs it's going to stop right there."

NOAA and the USGS monitor tsunamis using a series of deep sea bouys placed at strategic positions around the globe including off the California coast. That's because tsunamis have hit the state before. One of the largest, an Alaskan quake in 1964, created another one that damaged Pacifica, Half Moon Bay and other Northern California cities.

Though Northern California is not likely to experience a quake on the magnitude of the one near Japan, seismologists say it is vulnerable to devastating tsunamis. They point to the Cascadia subduction zone, a fault line near Eureka and the Canadian border. A shift in the ocean floor could cause a large tsunami to hit the Bay Area. Such an event last happened in the 1700s.